1. Field of the Invention
This invention relates to a sensitive silver halide photothermographic material for producing dye images.
2. Description of the State of the Art
A sensitive photothermographic material is a photographic material with which a visible image is produced by a method comprising two main steps (i) imagewise exposure of the material to actinic radiation (usually light) so as to form a latent image therein; and (ii) overall heating of the material to form an imagewise distribution of either the desired image substance or one or more compounds which readily provide that substance. Additional steps may be needed. For example, it may be necessary to transfer the image substance, or the compound or compounds to be used in providing that substance, to another layer.
A well known type of photothermographic material contains a photographic silver halide, which gives silver latent image specks on exposure, and an oxidation-reduction image-forming combination comprising a reducible organic silver salt and an organic reducing agent therefor. The latent image specks catalyze the reaction of the organic silver salt with the reducing agent to give visible image silver. For the overall heating step (ii) a temperature is chosen at which this reaction occurs at a convenient rate in the latent image areas but at a negligible rate in the background areas. Silver halide photothermographic materials of this kind are classified as System B by Carpeneter and Lauf, in Research Disclosure June 1978, page 9, Item No. 17029.
Various System B materials have been proposed which give an image of both dye and silver. In some of these materials, the dye is generated imagewise by reaction between a coupler, incorporated in the material and the oxidation product of the organic reducing agent (see, for instance, U.K. Patent Specification No. 1,400,244). In another proposed material, the dye is part of the reducible organic silver salt, and is rendered diffusible when that salt is reduced (see Research Disclosure May 1978, Item No. 16966). If it is necessary, or desirable, to separate the dye image from the silver image, one method which can be adopted is to transfer the dye image by diffusion to a receiving layer which contains a mordant for the dye. Diffusion transfer can be enabled, or assisted, by use of an elevated temperature and/or a solvent.
In any process involving a diffusion transfer step, particularly one carried out at an elevated temperature, it is very desirable for there to be a considerable difference between the diffusibility of the substance being transferred and that of the substance responsible for its generation or release. If the difference is inadequate, diffusion of reactants can occur in non-image areas leading to unwanted background density.